Methods of producing photoelectric devices

ABSTRACT

A photovoltaic device includes an intrinsic layer having two or more sublayers. The sublayers are intentionally deposited to include complementary concave and convex shapes. The sum of these layers resulting in a relatively flat surface for deposition of n- or p-doped layers. The photovoltaic device is optionally bifacial.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. non-provisionalpatent application Ser. No. 15/268,208 filed Sep. 16, 2016, thisapplication is a continuation-in-part of U.S. non-provisional patentapplication Ser. No. 15/263,031 filed on Sep. 12, 2016, this applicationis a continuation-in-part of U.S. non-provisional patent applicationSer. No. 15/267,979 filed on Sep. 16, 2016, this application also claimspriority to U.S. provisional application Ser. No. 62/369,923 filed Aug.2, 2016. The disclosures of these patent applications are herebyincorporated herein by reference.

BACKGROUND Field of the Invention

The invention is the field of photovoltaic devices.

Related Art

Photovoltaic devices include multiple layers of semiconductor materials.In devices that include heterojunctions these layers are often producedusing vacuum deposition techniques. A heterojunction is the interfacethat occurs between two layers or regions of dissimilar crystallinesemiconductors. These semiconducting materials have unequal band gaps asopposed to a homojunction.

SUMMARY

One of the difficulties in the manufacture of large area photovoltaicdevices is the deposition of layers that meet uniformity requirementsover the entire device. This is particularly a problem when multiplewafers are processed in a batch and/or when layers are particularlythin. For example, in many photovoltaic devices an “intrinsic layer” isused to passivate a wafer substrate prior to deposition of dopedsemiconductor layers. The thinness of the intrinsic layer is limited bythe uniformity of the deposition technique. If a deposition techniqueresults in more material being deposited in one area of the devicerelative to another area of the device, enough intrinsic layer must bedeposited such that proper coverage (e.g., passivation) is achieved evenin areas where the least amount of material is deposited. In variousembodiments of the invention, a deposition process including two or moreseparate steps is used to achieve a greater uniformity in depositionrelative to each layer considered individually. This approached isapplicable to both single wafer and batch wafer processing.

The invention includes an optionally bifacial photovoltaic device. Thedevice is typically a heterojunction device and includes a wafersubstrate, one or more “intrinsic” layer, and at least p- and n-dopedlayers. In various embodiments, the “intrinsic” layer, disposed betweenthe wafer substrate and the p- and n-doped layers, is formed itself frommultiple layers. These layers of the intrinsic layer are referred to asa first absorber layer and a second absorber layer, etc. These layersare intentionally formed such that the interface between them is curved,e.g., the amount of material in these layers is intentionallynon-uniform. However, the sum of the first and second absorber layers isless curved relative to their interface. The sum of two layers that areintentionally non-uniform is shown to result in an overall uniformitythat is greater than otherwise achieved.

The first absorber layer is deposited to produce a curved surface thatis either concave or convex. The second absorber layer is deposed suchthat its thickness changes in a complementary manner. For example, ifthe first absorber layer is convex then the second absorber layer isconcave. It has been demonstrated that with the precision and/oraccuracy at which a concave and convex surfaces can be deposited issufficiently greater than the precision and/or accuracy at which asingle flat surface can be deposited. Thus, the combination of the firstand second absorber layers produces a net flatter surface than thedeposition of a single “flat” surface.

In various embodiments, the division of an “intrinsic” layer into two ormore layers optionally provides additional advantages. For example, thecomposition of the two or more layers may be different.

Various embodiments of the invention include a photovoltaic devicecomprising: a wafer substrate; a first absorber layer disposed on afirst side of the wafer substrate; a second absorber layer, the firstabsorber layer being disposed between the wafer substrate and the secondabsorber layer, the first and second absorber layer sharing a firstcurved interface; a p-doped layer, a boundary between the secondabsorber layer and the p doped layer being flatter than the first curvedinterface; a first transparent conductive layer, the p-doped layer beingdisposed between the first transparent conductive layer and the wafersubstrate; front conductors in contact with the transparent conductivelayer; and rear conductors disposed on a second side of the wafersubstrate.

Various embodiments of the invention include a photovoltaic devicecomprising: a wafer substrate; a first absorber layer disposed on afirst side of the wafer substrate; a second absorber layer, the firstand second absorber layers in combination forming an intrinsic layer,the first absorber layer being disposed between the wafer substrate andthe second absorber layer, wherein a uniformity of a thickness of theintrinsic layer is greater than a uniformity of a thickness of the firstabsorber layer; a p-doped layer; a first transparent conductive layer,the p-doped layer being disposed between the first transparentconductive layer and the wafer substrate; front conductors in contactwith the transparent conductive layer; and rear conductors disposed on asecond side of the wafer substrate.

Various embodiments of the invention include a method of producing aphotovoltaic device, the method comprising: receiving a first wafersubstrate; depositing a first absorber layer on the first wafersubstrate, the deposition being performed to produce a controlledvariation in a thickness of the first absorber layer; depositing asecond absorber layer on the first absorber layer, the deposition beingperformed to produce a controlled variation in a thickness of the secondabsorber layer; depositing a p-doped layer on the second absorber layer,wherein the variation in the thickness of the first absorber layer andthe variation in the thickness of the second absorber layer incombination result in a first boundary between the second absorber layerand the p-doped layer being flatter than a second boundary between thefirst absorber layer and the second absorber layer; and depositing afirst transparent conductive layer on the p-doped layer.

Various embodiments of the invention include a method of producing aphotovoltaic device, the method comprising: receiving a first wafersubstrate; depositing a first absorber layer on the first wafersubstrate, the deposition being performed to produce a controlledvariation in a thickness of the first absorber layer; depositing asecond absorber layer on the first absorber layer, the deposition beingperformed to produce a controlled variation in a thickness of the secondabsorber layer, wherein a uniformity of the thickness of a firstabsorber layer is less than a uniformity of a sum of the thickness ofthe first absorber layer and the thickness of the second absorber layer;depositing a p-doped layer on the second absorber layer; and depositinga first transparent conductive layer on the p-doped layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a multilayer photovoltaic device, accordingto various embodiments of the invention.

FIGS. 2A, 2B and 2C include detailed illustrations of absorber layerswithin the multilayer photovoltaic device, according to variousembodiments of the invention.

FIGS. 3A and 3B illustrate detailed illustrations of multilayerphotovoltaic devices in a batch process, according to variousembodiments of the invention.

FIG. 4 illustrates methods of producing a multilayer photovoltaicdevice, according to various embodiments of the invention.

FIGS. 5A and 5B illustrate dopant concentration gradients, according tovarious embodiments of the invention.

DETAILED DESCRIPTION

FIG. 1 is an illustration of a multilayer Photovoltaic Device 100,according to various embodiments of the invention. Photovoltaic Device100 includes a Wafer Substrate 110 on which other layers are deposited.Wafer Substrate 110 is optionally a silicon wafer including etched orroughened surfaces. Wafer Substrate 110 includes materials configured toproduce charge pairs on the absorption of a photon of solar wavelengths.In various embodiments, Wafer Substrate 110 is at least 100, 125, 156 or200 millimeter (or any range between these values) in length along thelargest edge. Wafer Substrate 110 may have a pseudo square, round orpolygonal shape. Photovoltaic Device 100 is optionally produced in abatch process. Examples of the materials that may be included in WaferSubstrate 110 include, but are not limited to, Silicon, GalliumArsenide, Sapphire, and Silicon Carbide.

Photovoltaic Device 100 further includes a First Absorber Layer 115.First Absorber Layer 115 includes materials configured to passivate asurface of Wafer Substrate 110 and is at least partially transparent tosolar wavelengths. As used herein the term “solar wavelengths” isintended to include wavelengths of light that are significantlyenergetic to produce charge pairs in a photovoltaic material. Examplesof the materials that may be included in First Absorber Layer 115include, but are not limited to, un-doped silicon, hydrogenated silicon,silicon doped with phosphorus, boron, or any element in Group 13 (GroupIII) or Group 15 (Group V) of the periodic table.

First Absorber Layer 115 is intentionally produced in a manner thatresults in a controlled curvature of the surface of First Absorber Layer115 disposed opposite Wafer Substrate 110. This curvature is the resultof different amounts of material being deposited at different locations,and may be concave or convex. Examples of approaches that may be used toproduce such curvature include CVD (Chemical Vapor Deposition), ALD(Atomic Layer Deposition), and Epitaxial deposition. In variousembodiments, the difference between the maximum thickness and minimumthickness of First Absorber Layer 115 is at least 1, 5, 15, 20, 30 or 45Angstroms, or any range between these values. In various embodiments,the difference between the maximum thickness and minimum thickness ofFirst Absorber Layer 115 is at least 1, 3, 5, 10 or 30 percent of themaximum thickness. The minimum thickness of First Absorber Layer 115 maybe less than 5, 10, 50 or 100 angstroms, or any range between thesevalues. In embodiments in which First Absorber Layer 117 is deposited ina batch process the curvature may be present (and measured) over severalWafer Substrates 110. In various embodiments, in the batch process,parts of Wafer Substrate 110 for different Photovoltaic Device 100 maybe at least, 10, 15, 20, 25 or 50 cm from each other. The curvature maybe over at least these distances, or any range there between. In variousembodiments, at least 9, 12, 16, 20 or 25 wafer substrates are processedin a batch process in a same reaction chamber at the same time.

Photovoltaic Device 100 further includes a Second Absorber Layer 120. IfFirst Absorber Layer 115 is convex, then Second Absorber Layer 120 isconcave, and vice versa. The sum of First Absorber Layer 115 and SecondAbsorber Layer 120 can result in a surface of Second Absorber Layer 120(distal to Wafer Substrate 110) that is significantly more uniform inthickness, e.g., flatter, than the boundary and/or interface betweenFirst Absorber Layer 115 and Second Absorber Layer 120. In fact, whenproperly produced, the net uniformity in thickness achieved bydepositing two intentionally curved (non-uniform) layers can be greaterthan the uniformity achieved by the deposition of a single layer. TheSecond Absorber Layer 120 is deposited such that the First AbsorberLayer 115 is disposed between Wafer Substrate 110 and Second AbsorberLayer 120. In various embodiments, the thickness of the combined Firstand Second Absorber Layer 115 and 120 is uniform to within 1%, 3%, 5%,10% or 15%, or any range between these values, using the abovetechniques. This uniformity can be achieved over a single PhotovoltaicDevice 100 and/or over a plurality of Photovoltaic Device 100 producedin a batch process. For example, in various embodiments, this uniformityis achieved over a batch of Photovoltaic Devices 100 occupying an area(including gaps between devices) of at least 0.67, 1.375, 2.7 or 5.5,square meters.

FIGS. 2A, 2B and 2C include detailed illustrations of the First andSecond Absorber Layers 115 and 120 within the multilayer PhotovoltaicDevice 100, according to various embodiments of the invention. FIG. 2Aillustrates embodiments in which First Absorber Layer 115 is convex andSecond Absorber Layer 120 is concave. FIG. 2B illustrates embodiments inwhich First Absorber Layer 115 is concave and Second Absorber Layer 120is convex. The curvatures and dimensions illustrated in FIGS. 2A-2C aregreatly exaggerated for illustrative purposes. For example, the NetThickness 230 of the combined First Absorber Layer 115 and the SecondAbsorber Layer 120 is typically on the order of 20-200 angstroms. Whilethe Device Width 210 of Photovoltaic Device 100 can be on the order of100-200 millimeter. The Net Thickness 230 is approximately a sum ofThickness 215 and Thickness 220 at any particular point on the layers.In various embodiments, Net Thickness 230 varies by less than 1%, 3%, 5%or 10%. In various embodiments, Net Thickness 230 is less than 0.2, 2,10 or 20 angstroms. In various embodiments, the boundary between 2^(nd)Absorber Layer 120 and p-Doped Layer 125 is at least 5, 10, 25 or 50%more uniform than the boundary between 1^(st) Absorber Layer 115 and2^(nd) Absorber Layer 120.

FIG. 2C illustrates First and Second Absorber Layers 115 and 120 whereinthe curvature at their boundary is asymmetric. This situation, eitherconvex or concave, can be found in Photovoltaic Devices 100 produced atan edge of a batch (See FIGS. 3A and 3B).

A Surface 235 of First Absorber Layer 115 is adjacent to Wafer Substrate110. A Boundary 240 is disposed between First Absorber Layer 115 and theSecond Absorber Layer 120 and forms a curved interface. First AbsorberLayer 115 and the Second Absorber Layer 120 are optionally in directcontact at Boundary 240. A Surface 245 of Second Absorber Layer 120 isdistal to Wafer Substrate 110. The uniformity (e.g., flatness) ofSurface 245 is determined by the sum of the thicknesses of FirstAbsorber Layer 115 and the Second Absorber Layer 120. By properselection of deposition conditions, the flatness of Surface 245 may besignificantly greater than the flatness of Boundary 240. In other words,the sum of the First and Second Absorber Layers (115 and 120) may besignificantly more uniform in thickness relative to each of the Firstand Second Absorber Layers (115 and 120) considered separately.

Referring again to FIG. 1, Photovoltaic Device 100 further includes ap-Doped Layer 125 and a Transparent Conductive Layer 130. P-Doped Layer125 is disposed between Transparent Conductive Layer 130 and SecondAbsorber Layer 120. p-Doped Layer 125 includes a p-dopant configured toaccept electrons from Wafer Substrate 110, e.g., from charge pairsgenerated by absorption of a photon. Transparent Conductive Layer 130 istransparent to solar wavelengths and is configured to conduct electronscaptured by p-Doped Layer 125 to Metal Contacts 135. TransparentConductive Layer 130 optionally includes a metal oxide. Such p-dopedlayers and transparent conductive layers are well known in the art ofphotovoltaics. Example p-dopants include Boron, Aluminum, Nitrogen,Indium, Gallium, Tin and Lead. Seeding of Second Absorber Layer 120 witha dopant allows p-Doped Layer 125 to grow from nucleation sites.

The curvature of First Absorber Layer 115 and Second Absorber Layer 120are complementary, e.g., they are opposite in direction. As such, aboundary between Second Absorber Layer 120 and p-Doped Layer 125 isflatter than Boundary 240 between First Absorber Layer 115 and SecondAbsorber Layer 120. Further, as the uniformity of the combined First andSecond Absorber Layers 115 and 120 is improved relative to passivationusing a single intrinsic layer, it is possible to have a thinnerintrinsic layer and still assure passivation of the entire WaferSubstrate 110 surface. Further, adequate uniformity in net layerthickness can be maintained over a wider area. This permits larger scalebatch processing.

Photovoltaic Device 100 optionally further includes a Third AbsorberLayer 135 and a Fourth Absorber Layer 140. Third Absorber Layer 135 anda Fourth Absorber Layer 140 optionally have properties (e.g., structure,dimensions and materials) similar to First Absorber Layer 115 and SecondAbsorber Layer 120, respectively. Third Absorber Layer 135 and a FourthAbsorber Layer 140 are disposed on a side of Wafer Substrate 110opposite from First Absorber Layer 115. Third Absorber Layer 135 isdisposed between Wafer Substrate 110 and Fourth Absorber Layer 140. Acurved interface, similar to Boundary 240 is found between ThirdAbsorber Layer 135 and Fourth Absorber Layer 140. The uniformity inthickness of the combined Third and Fourth Absorber Layer 135 and 140 isgreater than the uniformity of either of these two layers individually.

Photovoltaic Device 100 typically further includes an n-Doped Layer 145.Third Absorber Layer 135 and a Fourth Absorber Layer 140 may or may notbe disposed between Waver Substrate 110 and n-Doped Layer 145. N-DopedLayer 145 includes an n-dopant configured provide electrons to electronsholes generated within Third Absorber Layer 135 and/or Fourth AbsorberLayer 140. A boundary between Fourth Absorber Layer 140 and n-DopedLayer 145 is optionally flatter than a boundary (e.g., shared interface)between Third Absorber Layer 135 and Fourth Absorber Layer 140. Examplen-dopants include Nitrogen, Phosphorus, Arsenic, Antimony, Bismuth andLithium. In addition Germanium, xenon, gold and platinum may be used inn-Doped Layer 145 and/or p-Doped Layer 125.

Photovoltaic Device 100 typically further includes a Second TransparentConductive Layer 150. Second Transparent Conductive Layer 150 isconfigured to conduct electrons between n-Doped Layer 145 and Back MetalContacts 155. Second Transparent Conductive Layer 150 may includematerials similar to those that can be included in First TransparentConductive Layer 130. In alternative embodiments Second TransparentConductive Layer 150 is replaced by a reflective conductive layer. Thisreflective conductive layer can include a metal such as copper, silver,aluminum or nickel, nickel vanadium, gold, platinum. The reflectiveconductive layer is configured to reflect light of solar wavelengthsback toward Wafer Substrate 110.

In some embodiments, First Absorber Layer 115 and Second Absorber Layer120 comprise the same materials. These materials can be at the same ordifferent concentrations. Alternatively, the concentrations of thematerials in First Absorber Layer 115 and Second Absorber Layer 120 maybe different. For example, Second Absorber Layer 120 may include a formof silicon having a different index of refraction relative to FirstAbsorber Layer 115. Second Absorber Layer 120 may include more or lessamorphous form of silicon (as compared to crystalline and measured byrefractive index) relative to First Absorber Layer 115. Second AbsorberLayer 120 may include a higher proton concentration relative to FirstAbsorber Layer 115. Second Absorber Layer 120 may include a seedmaterial, e.g., boron, configured to facilitate the deposition ofp-Doped Layer 125. This seed material optionally includes a p-dopantincluded in p-Doped Layer 125. The seed material may be deposited usingdiborane, B₂H₆, trimethyl/borate, and/or the like. In variousembodiments, Second Absorber Layer 120 and/or First Absorber Layer 115include a concentration gradient. For example, Second Absorber Layer 120may include a gradient of p-dopant material, the concentration of thep-dopant mater being less proximate to p-Doped Layer 125. Optionally,the Second Absorber Layer 120 includes a greater amount of p-dopantrelative to the First Absorber Layer 115. An example of a p-dopantgradient is discussed elsewhere herein.

In some embodiments, the differences between the discussion herein ofthe materials and structures of First Absorber Layer 115 and SecondAbsorber Layer 120 are optionally also applied to Third Absorber Layer135 and Fourth Absorber Layer 140, respectively. For example, ForthAbsorber Layer 140 may include a seed material configured to facilitatethe deposition of n-Doped Layer 145. This seed material can include ann-dopant material of n-Doped Layer 145.

In some embodiments the material differences between First AbsorberLayer 115 and Second Absorber Layer 120 make the material of the SecondAbsorber Layer 120 less transparent to solar wavelengths relative to thematerial of the First Absorber Layer 115. For example, in someembodiments it is desirable to add material to the Second Absorber Layer120 that make this layer more resilient to damage during the depositionof the p-Doped Layer 125. A cost of this resilience may be a reducedmaterial transparency. By forming the intrinsic layer from two distinctlayers (115 and 120), this cost can be confined to the Second AbsorberLayer 120.

FIGS. 3A and 3B illustrate detailed illustrations of multilayerPhotovoltaic Devices 100 in a batch process, according to variousembodiments of the invention. In such a process, more than onePhotovoltaic Device 100 is produced on a single Platform 310. Platform310 may be configured to hold at least 9, 12, 16, 20, 25, 64, 100, or210 Photovoltaic Devices 100 at once, (or any range between thesevalues). Each of these Photovoltaic Devices 100 typically include aseparate Wafer Substrate 110, that are processed in a batch process atthe same time. The Photovoltaic Devices 100 may be of any shape and canbe arrayed in a square as shown or in any appropriate packing pattern.During production of Photovoltaic Devices 100 on Platform 310, maximumdistances between parts of the produced Photovoltaic Devices 100 may beas least 0.1, 1, 5, 10, 50 or 100 millimeter, or any range between thesevalues. In various embodiments, these distances are maintained whilemaintaining a batch uniformity in thickness of the combined First andSecond Absorber Layers 115 and 120 of at least 1%, 2%, 5% or 10%. Asused herein, the term “batch uniformity” is used to specify theuniformity in thickness of the sum these layers over all thePhotovoltaic Devices 100 in a batch. In contrast, “wafer uniformity” isused to specify the uniformity of the thickness of the sum of theselayers over a single Photovoltaic Device 100. Multiple PhotovoltaicDevices 100 are optionally disposed on Platform 310 during deposition ofany one or more of the various layers illustrated in FIG. 1. In variousembodiments, the average total thickness of the First and SecondAbsorber Layers (115 & 120) is less than 30, 40, 50, 55 or 60 angstroms.This average may be over a single Photovoltaic Device 100 or a batchthereof.

Wafer uniformity is determined by measuring thickness of a layer atseveral points on a wafer and then calculating the wafer uniformity toby the formula:((Max Thickness)−(Min Thickness))/(2×Average Thickness)In some industry standards the thickness at a minimum of 5 points ismeasured. In other approaches 9 or 13 points are measured. Possiblemeasurement point Locations 340 are illustrated in FIG. 3A with an “X”.The uniformity values recited herein are based on measurements at theseexample locations, or similar locations adapted for Wafer Substrates 110of different shapes. Batch uniformity is calculated by first determiningthe wafer uniformity for each Photoelectric Device 100 in a batch. Thedifference between the maximum wafer uniformity and the minimum waferuniformity is then divided by two times the average wafer uniformity (inan equation similar to that shown above).

When Photovoltaic Devices 100 are produced in a batch, the variation inlayer deposition thickness may vary over the entire batch. The Curves320A and 320B are graphic illustrations of such variation. Curve 320Aillustrates a concave deposition function in which relatively greaterthickness is found at the edges of a batch. Curve 320B illustrates aconvex deposition function in which a relatively greater thickness isfound near the center of a batch. (The heights of the illustrated Curves320A and 320B are greatly exaggerated for illustrative purposes,relative to the Photovoltaic Devices 100 shown in FIG. 3B.) Thesedeposition functions may produce First and Second Absorber Layers 115and 120 such as illustrated in FIG. 2C. The two complementary depositionfunctions (of First and Second Absorber Layers 115 and 120) provide agreater batch uniformity in the thickness of an intrinsic layer overseveral Photovoltaic Devices 100, as compared to deposition of anintrinsic layer in a single step.

FIG. 4 illustrates methods of producing multilayer Photovoltaic Device100, according to various embodiments of the invention. The produceddevice may be single- or bifacial. As is discussed elsewhere herein, anintrinsic layer is produce using two or more separate absorber layers.These two or more layers are typically produced in distinct steps.

In a Receive Substrate Step 410, Wafer Substrate 110 is received. WaferSubstrate 110 may be etched to produce a rough surface for lightentrapment.

In a Deposit 1^(st) Absorber Layer Step 415, First Absorber Layer 115 isdeposited on Wafer Substrate 110. The deposition is typically performedusing techniques known in the art of semiconductor manufacturing. Forexample, CVD, ALD or Epitaxial deposition. First Absorber Layer 115 isintentionally deposited to form a concave or convex shape. This shapemay be present over a single Photovoltaic Device 100 and/or a batch ofPhotovoltaic Devices 100. The shape (convex or concave) of FirstAbsorber Layer 115 is reproducibly controlled.

In a Deposit 2^(nd) Absorber Layer Step 420, Second Absorber Layer 120is deposited such that First Absorber Layer 115 is disposed betweenWafer Substrate 110 and Second Absorber Layer 120. Second Absorber Layer120 is intentionally deposited to form a convex or concave shape that iscomplementary to the same of First Absorber Layer 115. The variation inthe thickness of Second Absorber Layer 120 is controlled to produceSurface 245 on the side of Second Absorber Layer 120 distal to WaferSubstrate. The flatness of Surface 245 is dependent on the uniformity inthickness of the combined First and Second Absorber Layers 115 and 120.The thickness of the combined First and Second Absorber Layers 115 and120 is optionally uniform as measured over an entire batch ofPhotovoltaic Devices 100.

Second Absorber Layer 120 is optionally deposited using differentconditions than the deposition of First Absorber layer 115. Thecondition differences can include temperature, pressure, excitationpower, gas glows, gas distribution and plate to substrate surfacespacing.

In some embodiments Photovoltaic Device 100 is moved between reactionchambers between Step 415 and Step 420. In some embodiments, a reactionchamber within which Step 415 occurs is evacuated prior to Step 420.Step 420 is then performed using different conditions as discussedelsewhere herein.

In a Deposit p-Doped Layer Step 425, p-Doped Layer 125 is deposited onSecond Absorber Layer 120. The variation in the thickness of FirstAbsorber Layer 115 and in the thickness of the Second Absorber Layer 120results in a boundary at Surface 245 that is flatter than Boundary 240between the First Absorber Layer 115 and the Second Absorber Layer 120.As noted above Surface 245 is at an interface between p-Doped Layer 125and Second Absorber Layer 120. A seed layer (not shown) is optionallydeposited between p-Doped Layer 125 and Second Absorber Layer 120.

In a Deposit 1^(st) Conductive Layer Step 430, First TransparentConductive Layer 130 is deposited on p-Doped Layer 125. As describedelsewhere herein, First Transparent Conductive Layer 130 is preferablyhighly transparent to solar wavelengths.

In a Deposit 3^(rd) Absorber Layer Step 435, Third Absorber Layer 135 isdeposited on a side of Wafer Substrate 110 opposite First Absorber Layer115. In an optional Deposit 4^(th) Absorber Layer Step 440, FourthAbsorber Layer 140 is deposited on Third Absorber Layer 135. Thedeposition of Third Absorber Layer 135 and Fourth Absorber Layer 140 areperformed using methods similar to those used in Deposit 1^(st) AbsorberLayer 415 and Deposit 2^(nd) Absorber Layer 420. For example, theselayers are deposited to produce curved distributions in thickness foreach layer, which results in a more uniform total thickness. Thedeposition of Fourth Absorber Layer 140 is performed such that a side ofFourth Absorber Layer 140 proximate to n-Doped Layer 145 is flatter thana side of the fourth Absorber Layer 140 proximate to Third AbsorberLayer 135. Typically, the thickness of the combination of Third AbsorberLayer 135 and Fourth Absorber Layer 140 is more uniform than thethickness of Third Absorber Layer 135 alone.

In a Deposit n-Doped Layer Step 445, n-Doped Layer 145 is deposited on aside of Wafer Substrate 110 opposite p-Doped Layer 125. This depositionmay be performed using various techniques described herein.

In a Deposit 2^(nd) Conductive Layer Step 450, Second TransparentConductive Layer 150 is deposited adjacent to n-Doped Layer 145. Inalternative embodiments, Transparent Conductive Layer 150 includes alayer that is more reflective than transparent to solar wavelengths.

In an Add Contacts Step 455, Metal Contacts 135 and 155 are added on thelayers deposited in Steps 430 and 450. These Metal Contacts 135 and 155enable electrical connections to Photovoltaic Device 110.

FIGS. 5A and 5B illustrate a dopant concentration gradient, according tovarious embodiments of the invention. Example, concentrations of p- andn-dopant materials in p-Doped Layer 125, Second Absorber Layer 120,n-Doped Layer 145 and Fourth Absorber Layer 140 are shown.

Several embodiments are specifically illustrated and/or describedherein. However, it will be appreciated that modifications andvariations are covered by the above teachings and within the scope ofthe appended claims without departing from the spirit and intended scopethereof. For example, the disclosed approach to improving the uniformityof layers may be applied to device layers other than the intrinsiclayers of photovoltaic devices. Additional minor intermediate layers maybe disposed between those illustrated in FIG. 1. References todepositing a first layer “on” a second layer may include suchintermediate layers between the first and second layers—the first andsecond layers are still considered “on” each other. Finally, thetechniques disclosed herein may be applied to other types of devicessuch as displays and imaging semiconductors. These techniques areoptionally used to produce these products in larger batches than wouldotherwise be economical. While Wafer Substrate 110 is typically flat, insome embodiments a curved device is produced by starting with a curvedsubstrate. In these embodiments the “flatness” of a layer is consideredthe degree of match between that layer and the shape of the substrate.Further, while the examples provided herein include a p-doped layer onthe “front” of the device and an n-doped layer on the “back,” thelocation of these layers, their respective dopants and the order ofsteps, may be reversed.

The embodiments discussed herein are illustrative of the presentinvention. As these embodiments of the present invention are describedwith reference to illustrations, various modifications or adaptations ofthe methods and or specific structures described may become apparent tothose skilled in the art. All such modifications, adaptations, orvariations that rely upon the teachings of the present invention, andthrough which these teachings have advanced the art, are considered tobe within the spirit and scope of the present invention. Hence, thesedescriptions and drawings should not be considered in a limiting sense,as it is understood that the present invention is in no way limited toonly the embodiments illustrated.

What is claimed is:
 1. A method of producing a photovoltaic device, themethod comprising: receiving a first wafer substrate; depositing a firstabsorber layer on the first wafer substrate, the deposition beingperformed to produce a controlled variation in a thickness of the firstabsorber layer; depositing a second absorber layer on the first absorberlayer, the deposition being performed to produce a controlled variationin a thickness of the second absorber layer, wherein a uniformity of thethickness of a first absorber layer is less than a uniformity of a sumof the thickness of the first absorber layer and the thickness of thesecond absorber layer; depositing a first doped layer on the secondabsorber layer; depositing a first transparent conductive layer on thefirst doped layer depositing a third absorber layer on a side of thewafer substrate opposite the first absorber layer; depositing a fourthabsorber layer on the third absorber layer; and depositing a seconddoped layer on the fourth absorber layer, wherein deposition of thefourth absorber layer is performed such that a side of the fourthabsorber layer proximate to the second doped layer is flatter than aside of the fourth absorber layer proximate to the third absorber layer.2. The method of claim 1, further comprising depositing a secondtransparent conductive layer on the second doped layer.
 3. The method ofclaim 1, further comprising depositing a reflective transparentconductive layer on the second doped layer.
 4. The method of claim 1,wherein the first doped layer is p-doped and the second doped layer isn-doped.
 5. The method of claim 1, wherein the first doped layer isn-doped and the second doped layer is p-doped.
 6. The method of claim 1,wherein the second absorber layer has a greater refractive index thanthe first absorber layer.
 7. The method of claim 1, wherein the firstabsorber layer is deposited at conditions to make the first absorberlayer concave.
 8. The method of claim 1, wherein the first absorberlayer is deposited at conditions to make the first absorber layerconvex.
 9. The method of claim 1, further comprising texturing thesubstrate wafer.
 10. A method of producing a photovoltaic device, themethod comprising: receiving a first wafer substrate; depositing a firstabsorber layer on the first wafer substrate, the deposition beingperformed to produce a controlled variation in a thickness of the firstabsorber layer; depositing a second absorber layer on the first absorberlayer, the deposition being performed to produce a controlled variationin a thickness of the second absorber layer, wherein a uniformity of thethickness of a first absorber layer is less than a uniformity of a sumof the thickness of the first absorber layer and the thickness of thesecond absorber layer; depositing a first doped layer on the secondabsorber layer; depositing a first transparent conductive layer on thefirst doped layer; and receiving at least a second wafer substrate,wherein the step of depositing the first absorber layer is performed onboth the first and second wafer substrates at the same time.
 11. Themethod of claim 10, wherein a part of the first wafer substrate is atleast 15 cm from a part of the second wafer substrate during the step ofdepositing the first absorber layer.
 12. The method of claim 10, whereinthe first and second wafer substrates are disposed on a same platformduring the step of depositing the first absorber layer.
 13. The methodof claim 12, wherein the platform is configured to hold at least 9 wafersubstrates during the step of depositing the first absorber layer. 14.The method of claim 1, wherein the controlled variation in a thicknessof the first absorber layer includes a controlled variation in thicknessacross multiple wafer substrates.
 15. The method of claim 14, whereinthe controlled variation across multiple wafer substrates is over alength of at least 20 cm.
 16. The method of claim 1, wherein the secondabsorber layer includes a seed material configured for depositing thefirst doped layer.
 17. The method of claim 1, wherein the first dopedlayer is p-doped or n-doped.
 18. The method of claim 1, wherein thefirst absorber layer is deposited to be convex and the second absorberlayer is concave.
 19. The method of claim 1, wherein the first absorberlayer is deposited to be concave and the second absorber layer isconvex.